A crystalline thermoplastic polyester resin (e.g. a polyalkylene terephthalate resin), which has not only excellent mechanical properties, electrical properties, and other physical and chemical characteristics but also good processability, has been in use as an engeneering plastic for a wide range of products such as cars and vehicles, elecric or electronic parts.
Such a crystalline thermoplastic polyester resin can be used for various moulded articles by itself. For some applications, however, various reinforcing agents or additives are incorporated into the resin to improve its properties, in particular its mechanical properties. Where a high mechanical strength and rigidity is required, fibrous reinforcing agents, a glass fibre and a carbon fibre to name a few, are added to the crystalline thermoplastic resin. For a high dimentional stability and surface smoothness or surface properties, there has been a practice of adding a noncrystalline polymer such as a styrene-butadiene-acrylonitrile copolymer (ABS resin) to the polyester resin.
Required characteristics for the moulded article are getting higher and higher. For example, further improvement is hoped in sliding properties. A composition containing the fibrous reinforcing agent and a composition containing the noncrystalline polymer have a high mechanical strength and dimentional stability. However, once they are rubbed along with an article of a metal or a resin counterpart member, both of the resin compositions and the counterpart member (if the member is a resin) are worn out with creaking sounds only to deteriorate the sliding properties of the compositions. A moulded article which is made of a polyester incorporated with a noncrystalline polymer such as an ABS resin shows a poorer abrasion resistance with respect to the counterpart member of a resin than a moulded article comprising a polyester alone, and wears itself out to a great extent and spreads its worn-out powder.
As a solution to these problems, a grease has been conventionally applied to a sliding site. However, the performance can be declined once a grease runs out during a long operation period. By way of illustration, in the case of an information recording medium which demands high precision (e.g. a compact disc (CD), a read only memory (ROM) and other devices with high precision), attachment of a grease on the CD may cause a read error. Thus, improvement of slidability is a matter of great importance.
In order to enhance sliding properties, there has been suggested a composition blended with an agent for improving sliding characteristics including a PTFE (polytetrafluoroethylene) powder and a graphite (a black lead). Such a composition shows a slightly improved coefficient of friction, but its abrasion properties are not improved, rather on the further decline. Addition of the agent for improving sliding characteristics in a large amount, which is aimed at an achievement of a low friction coefficient, results in sacrificing the mechanical characteristics such as mechanical strength and toughness. Further improvements have been thus required.
Japanese Patent Application Laid-open No. 76048/1992 (JP-A-4-76048) discloses sliding parts which are made of a resin composition comprising a polybutylene terephthalate or a polyethylene terephthalate, a graft copolymer such as an ABS resin, and an inorganic filler, with an intention of improving friction/abrasion characteristics and preventing a creak or squeak while sliding.
Japanese Patent Application Laid-open No. 150022/1995 (JP-A-7-150022) discloses a polyester resin composition with a good slidability which comprises a crystalline thermoplastic polyester resin, an olefinic copolymer composed of an olefinic polymer fragment and a vinyl-series polymer fragment, and a fatty acid ester obtainable from a fatty acid having 12 or more carbon atoms.
Despite all these attempts, it is still difficult to enhance the friction/abrasion characteristics of the above-mentioned resin composition to a great extent while retaining the mechanical characteristics at a high level.